A large-scale security crackdown by Iraqi and US forces has begun in Baghdad. Some 40,000 troops are taking part in the operation, which is being billed as the biggest security clampdown in the Iraqi capital since the US-led invasion of 2003. It is aimed at putting pressure on militants after the killing of al Qaeda’s leader in Iraq, Abu Musab al-Zarqawi, last week.The number of checkpoints and patrols has been increased, particularly in districts populated by Iraq’s Sunni Muslim minority. It comes just a day after the US President made a surprise appearance in Baghdad. Today thousands of supporters of the radical Shi’ite cleric Muqtada al-Sadr marched in the north of the city, denouncing the visit by George W. Bush and calling for the withdrawal of US forces. US troops have been buoyed by recent successes, in particular the killing of al-Zarqawi. But with the almost daily carnage set to continue, and the US presence as unpopular as ever, the military faces a monumental task to gain full control of Baghdad’s streets.